3.5 Selection Expressions

Selection expressions, also known as “sexes” in recutils
jargon, are infix expressions that can be applied to a record.
A “sex” is a predicate which selects a subset of records within a recfile.
They can be simple expressions involving just one operator and a pair of
operands, or complex compound expressions with parenthetical sub-expressions
and many operators and operands.
One of their most common uses is to examine records matching a particular
set of conditions.